1. Field of the Invention
This invention relates to generating signals having a desired frequency.
2. Description of the Related Art
Many approaches have been utilized to provide clock signals for modern electronic systems. In a crystal-less clock generator system, which needs to produce a signal with a precise frequency, but without the use of a crystal oscillator, one approach uses temperature compensation to adjust an output of a controlled oscillator, e.g., a voltage controlled oscillator (VCO), based on measured temperature. Such an approach essentially records the exact settings of the VCO fine tuning controls as a function of measured frequency that result in the correct output frequency at a particular temperature. Because both fine and coarse VCO tuning controls are generally not sufficiently accurate or temperature insensitive, accuracy is achieved only when the generated frequency is precisely the same as was used during the production test measurements. Therefore, parts using such an approach are tuned in production test for a particular frequency, and cannot produce any other frequency accurately.
In another prior art system, a crystal-based reference clock, which is not precise and is somewhat temperature dependent, is used to tune a controlled oscillator to produce an output clock. In this case, the nominal frequency (and even temperature sensitivity) of the crystal reference can be learned in test, then an arbitrary desired output clock can be produced using a fractional-N phase-locked loop (PLL) or frequency-locked loop (FLL). However, that approach requires that two oscillators, the reference oscillator and the controlled oscillator, exist and be constantly powered, thus incurring substantial hardware area and power costs in addition to the possibility of spurs arising from energy at the reference clock frequency leaking into the output clock.